A wheel and a braking rotating member of a vehicle are rotatably supported with respect to a suspension device by a wheel supporting rolling bearing unit. Upon cornering of the vehicle, a large moment is applied to the wheel supporting rolling bearing unit. Thus, in order to maintain stability during the cornering, high moment rigidity needs to be ensured for such a wheel supporting rolling bearing unit. Accordingly, a configuration in which rolling elements are arranged in double rows and preload as well as back-to-back duplex type contact angles are given to the respective rows of the rolling elements has generally been adopted in conventional rolling bearing units for wheel support. Further, in recent years, in order to ensure high moment rigidity without increasing a size, there have been proposed some configurations as disclosed, for example, in Patent Documents 1 to 5 in which pitch circle diameters or rolling element diameters are designed to be different between the respective rows of rolling elements.
FIG. 4 shows a configuration disclosed in Patent Document 4. This wheel supporting rolling bearing unit 1 includes a hub 2, an outer ring 3 and a plurality of balls 4, 4 which are rolling elements. The hub 2 includes a combination of a hub body 5 and an inner ring 6. The hub 2 is formed with an attaching flange 7, by which a wheel and a braking rotating member is supported, on an axially outer side portion thereof, and double rows of inner ring raceways 8a, 8b on an axially middle portion and an axially inner side portion thereof respectively (in the following description and the claims, the axially outer side is an outer side in a width direction of an assembled vehicle, i.e. the left side in the drawings, and the axially inner side is a central side in the vehicle width direction, i.e. the right side in the drawings). Between the inner ring raceways 8a, 8b, a diameter of the inner ring raceway 8a on the axially outer side is larger than a diameter of the inner ring raceway 8b on the axially inner side. Base ends of a plurality of studs 15 are fixed to the attaching flange 7 so that the braking rotating member, such as a disc, or the wheel can be fixedly supported by the attaching flange 7.
In order to make the diameters of the respective inner ring raceways 8a, 8b different from each other, in the configuration shown in FIG. 4, an outer circumferential slanted step 9 is formed on an outer circumferential surface of an axially middle portion of the hub body 5 which is slightly on an axially inner side of the inner ring raceway 8a on the outer side. The outer circumferential slanted step 9 is slanted such that an outer diameter thereof gradually decreases toward the axially inner side direction. In addition, a smaller-diameter step 10 is formed on an axially inner side portion of the hub body 5 which is on an axially inner side of the outer circumferential slanted step 9. The inner ring raceway 8b on the axially inner side is formed on an outer circumferential surface of the inner ring 6. The inner ring 6 is fitted on the smaller-diameter step 10, and the inner ring 6 is fixedly coupled to the hub body 5 by a riveted portion 11 formed on an axially inner end portion of the hub body 5. Each of the inner ring raceways 8a, 8b has an arcuate cross section (a generatrix shape), and the outer diameters of the respective inner ring raceways 8a, 8b decrease as they become close to each other (toward the axially middle portion of the hub 2).
The outer ring 3 includes double rows of outer ring raceways 12a, 12b formed on an inner circumferential surface thereof and a coupling flange 13 formed on an outer circumferential surface thereof for fixedly coupling the outer ring 3 to a suspension device. Between the outer ring raceways 12a, 12b, a diameter of the outer ring raceway 12a on the axially outer side is larger than a diameter of the outer ring raceway 12b on the axially inner side. Accordingly, in the configuration shown in FIG. 4, an inner circumferential slanted step 14 is formed on an inner circumferential surface of an axially middle portion of the outer ring 3 which is on an axially inner side of the outer ring raceway 12a on the axially outer side. The inner circumferential slanted step 14 is slanted such that an inner diameter thereof decreases toward the axially inner side direction. Each of the outer ring raceways 12a, 12b has an arcuate cross section (a generatrix shape), and the inner diameters of the respective outer ring raceways 12a, 12b decrease as they become close to each other (toward the axially middle portion of the hub 2).
Between the inner ring raceways 8a, 8b and the outer ring raceways 12a, 12b, the balls 4, 4 are rollably arranged respectively. In this state, preload as well as back-to-back duplex type (DB type) contact angles are given to the respective balls 4, 4 arranged in double rows. Pitch circle diameters of the respective rows of balls 4, 4 are different from each other in accordance with the difference of the diameters between the inner ring raceways 8a, 8b and between the outer ring raceways 12a, 12b. That is, a pitch circle diameter PCDOUT of the balls 4, 4 in the axially outer side row (outboard row) is larger than a pitch circle diameter PCDIN of the balls 4, 4 in the axially inner side row (inboard row) (PCDOUT>PCDIN). Although, the balls 4, 4 are used as the rolling elements in the example shown in the drawing, tapered rollers may be used as the rolling elements in case where a rolling bearing unit is for a heavy-weight vehicle.
Configurations of the wheel supporting rolling bearing unit disclosed in Patent Documents 1 to 5, in which pitch circle diameters of the rolling elements in respective rows are different from each other, are as described above. According to such a configuration, moment rigidity can increased in accordance with an increase of the pitch circle diameter PCDOUT of the outboard row, whereby a design for improving running stability during the cornering and durability of the wheel supporting rolling bearing unit can be easily made. The pitch circle diameter PCDIN of the inboard row, on the other hand, does not need to be increased. Thus, a diameter of a portion (an attachment hole of a knuckle) of the suspension device does not need to be increased in particular. Accordingly, the running stability and the durability can be improved without increasing a size of such a portion of the suspension device.
Patent Document 6 discloses a configuration in which pitch circle diameters of the rolling elements in respective rows are equal to each other, further in which thermally-hardened layers are separately or continuously formed in portions including outer ring raceways.    Patent Document 1: JP 2003-232343 A    Patent Document 2: JP 2004-108449 A    Patent Document 3: JP 2004-345439 A    Patent Document 4: JP 2006-137365 A    Patent Document 5: WO/2005/065077    Patent Document 6: JP 3-22124 U